The present invention relates generally to reception control with respect to chargeable channels in a community antenna television (hereinafter abbreviated to CATV) system, and particularly to a system for mixing jamming signals into CATV signals in frequency bands of channels which are not permitted to be viewed (hereinafter simply referred unpermitted channels) so as to scramble the CATV signals in the specified channels in subscriber side CATV signal distributing systems.
Conventionally, a CATV system can be used for providing changeable programs for subscribers, and various systems have been realized to prevent viewing of unpermitted channels. Here, the term "chargeable program" means reception of a specified channel determined on the basis of a contract for an individual subscriber beyond the basic service determined by the contract with the subscriber.
The simplest arrangement to prevent a person from unauthorized viewing of chargeable programs is one in which a frequency converter for the CATV signal is provided at each subscriber's terminal for converting the frequency band of the CATV signal into that of a VHF channel so that the CATV signal can be received by a TV set, wherein a band-elimination filter is provided in the coaxial cable leading to the subscriber's terminal for preventing the subscriber from receiving channels other than the contracted ones.
In this system, however, there have been such disadvantages that it is necessary to change the band elimination filter (hereinafter referred to as a trap) every time there occurs a change in the contracted channels, resulting in a considerable cost. Also, the number of traps which can be used at any one location is limited so that the number of the changeable channels is limited correspondingly.
In order to eliminate these disadvantages, addressable subscribers' terminals have been used. The arrangement of such a terminal will be described hereunder.
In this system, each subscriber's terminal is subject to polling by the program broadcasting station (hereinafter referred to as a center), wherein an address stored in advance in the terminal is compared with a polling address. If the two values coincide, viewing is allowed. Hence, reception control can be performed by data communications from the center. The subscriber terminal is therefore provided with a demodulator and a microprocessor for reading and processing the received data.
Further, with respect to the chargeable programs, a method has been employed whereby the CATV signal of a chargeable program is scrambled in advance at the center, and a control signal in a special CATV channel is descrambled at the subscriber's terminal. Typically, in such a channel scrambling system, horizontal synchronizing signals of the scrambled TV signal are eliminated at the center so that it is difficult to perform synchronized reproduction at an unauthorized viewer's TV set. Accordingly, it is necessary to restore the synchronizing signals for subscribers authorized to receive such signals, and therefore it is required to provide a descrambler and a demodulator for receiving data for controlling the descrambler and the demodulator at the subscriber's terminal.
In the conventional unauthorized viewing preventing systems described above, it is necessary to provide a descrambler and a demodulator for data communication at each subscriber's terminal, and hence such systems are expensive. Further, there have been disadvantages in that the quality of the received picture is deteriorated in the scrambling and descrambling process. Moreover there is still a possibility for an unauthorized subscriber to receive chargeable programs by reconstructing the terminal.
Further, in consideration of the foregoing disadvantages, there has been proposed an unauthorized viewing preventing system in which time-division type jamming signals for the chargeable channels are produced at various distribution points in the system, which jamming signals are selectively supplied to CATV signals in unauthorized channels.
In such a system in which jamming signals are applied to the chargeable channel signals and mixed therewith for jamming purposes, it has generally been the practice to provide the jamming signals using one or more voltage-controlled oscillators (VCOs) and phase-locked loops. The voltage-controlled oscillators are periodically stepped through the frequencies of the various CATV channel signals to be jammed. That is, the jamming signals are provided on a time-divisional basis.
In order to provide effective jamming, it is necessary that the jamming signal not only be of a sufficient amplitude, but it must also have a sufficient repetition period and accuracy of frequency so as to disturb the horizontal and vertical synchronizing signals to the extent that it is impossible to receive an unauthorized channel. However, it has been found difficult to attain this using conventional circuitry in that the lock-in time and synchronizing time of the phase-locked loop is not negligible. Accordingly, the level of the scrambling function or the number of channels which can be scrambled in this manner is limited.
Further, in order to provide a phase-locked loop system which can operate at a high-speed, it is necessary to employ as a reference signal a signal of a high-frequency. This makes the circuitry expensive.
Still further, if a jamming signal generator of this type is provided for every channel, the overall system cost is quite high. Moreover, using conventional circuitry, the output frequency of the voltage-controlled oscillator tends to vary outside the band of the signal to be jammed prior to lock-in. This causes an unwanted influence in channels which are not to be scrambled. Yet further, it has proven difficult to control the change over switch used to control jamming signal mixing operations.